In my dojo we have the saying, "If you can't teach it then you aren't doing it right". One of the best ways to master a technique is to teach it to someone else so that it really becomes a part of you.
I learn a lot simply by taking ukemi from my sempai, too, so I know how it's supposed to feel.
I feel the same way. Even as a beginner per-say, I have helped instruct during the "intro to Aikido" classes and find that I learn more about a specific technique when I try to teach it to kaohai. (Notice I said "try"
As for the topic at hand.
I enjoy working with less experienced students because as mentioned before, it slows me down.
I also enjoy working with experienced Sempai because with each one, I get a different perspective to the technique as well as a take on how well I am learning it.
In addition, each Sempai at my dojo have completely different personalities so that helps learn with a different perspective in mind. It's kind of a crap-shoot as to how aggressive they will teach as well. Sometimes far within my limits and sometimes stretching that a bit which is refreshing actually. I thrive on it.